In recent years substantial effort has been directed to the formation of plastic bottles as a partial replacement of glass bottles. This effort has built on knowledge that plastics, such as polyethylene terephthalate (PET), when molecularly stretched are tougher but lighter than glass; and on the glass art which describes both methods and apparatus for stretching and blowing glass in both the longitudinal and transverse directions.
For formation of the plastic container, it is necessary to start with a parison which is then heat-treated and stretch-blown into the plastic container. In the final plastic container, such as a bottle, it is desirable to have uniform wall thickness free of unsightly defects, both from the stand point of wall strength and visual appeal to the ultimate purchaser of the bottle. In order to obtain a bottle having uniform wall strength (thickness) free of unsightly defects, it is necessary to start with a substantially perfect parison. Accordingly, the parison cannot have variations in wall thickness or contain defects such as will be obtained if the parison mold used in forming the parison contains voids or is otherwise defective due to improperly filled molds, shrinkage in the mold, or the like. Moreover, in recent years the speed of formation of the parison, as well as the plastic bottles, has become critical in order to provide a viable commercial operation. This speed of operation requires the continuous formation of a plurality of parisons simultaneously, utilizing a single injection mechanism.
There are a substantial number of patents disclosing apparatuses for injection molding of a molding material which include an accumulator in the injection assembly. These apparatuses are designed and the intent is to obtain parts, such as parisons in a multi-cavity mold, which are free from defect and which provide parts having uniform wall thicknesses. However, all of the known injector nozzle valve and accumulator assemblies suffer disadvantages from the standpoint of speed and/or the defects in the formed parts caused by a failure of the accumulator cylinder to be purged completely after each operation, resulting in the injection of old plastic melt into the mold with subsequent operation resulting in wall defects and/or in not providing an adequate holding pressure on the mold cavity with the addition of new material. If there is no holding pressure, the mold cavity as a result of shrinkage can provide an article having non-uniform wall thickness, or the like, in the formed article.